Hello Liam,
My first recommendation is to use symmetry boundary condition by splitting the flow domain with respect to XZ plane, unless you will have a side slip flow. This will help you save significant amount of resources. Second recommendation, start with a mesh that is as coarse as possible, then start adding each refinement one by one. This way it will be easier to troubleshoot the error. You can even perform a mesh independency study.
Some other remarks: you may want to consider enlarging your far field geometry, and this will help flow to develope properly. Looking at the vehicle, please also be aware that Incompressible simulation type is supported for Mach numbers that are up to 0.3. In case you need a higher speed, you may want to switch to compressible simulation type. Compressible simulations should also give stable results with Mach numbers that are up to 0.6-0.7. (before the shocks are generated on wing surfaces.)
Hope these are helpful,
Kaan.